Imprinting in Mammals Flashcards
What was the early evidence that paternal genomes are not functionally equivalent?
Hybrids
- F horse + M donkey = Mule (strong +viable)
- M horse + F donkey = Hinny (useless)
- M Lion +F tiger = Liger (huge)
- F lion+ M tiger = tigon (sterile males)
What do the hybrids highlight?
mating strategies influence parental gene expression
- lions and tigers would never normally be in contact
What is the differences in mating strategies between tigers and lions that cause the excessive growth in ligers?
- Male lions promote cub growth via IGF-2 to enhance competitive advantage.
- Female lions suppress IGF-2 to ensure equal-sized cubs for balanced survival.
- Female tigers lack the IGF-2 suppression mechanism.
- When a male lion mates with a female tiger, IGF-2 remains unchecked, leading to excessive growth in ligers.
What was the study that showed both maternal and paternal genes are essential for embryo development?
** Early studies in mice
- Viable embryo: Oocyte (egg) with both maternal and paternal haploid pronuclei.
- Non-viable embryo: Oocyte with two maternal pronuclei (gynogenetic diploid).
- Non-viable embryo: Oocyte with two paternal pronuclei (androgenetic diploid).
How did scientists know that the non-viable embryos were not due to sex?
- even XX individuals did not survive
- The issue was not due to sex chromosomes, but rather genomic imprinting.
Where is IGF-II imprinted?
- Insulin-like Growth Factor II (IGF-II) gene is maternally imprinted in mice
- means only the paternal copy is active
What was the set up and results for the IGF-II mice experiment?
Wild-type female × Heterozygous KO IGF-II male
→ Some offspring were growth-retarded if they inherited the KO allele from the father.
Heterozygous KO IGF-II female × Wild-type male
→ All offspring were normal-sized because the maternal copy is always silenced.
What was the conclusion of the IGF-II mice experiment?
- Only the paternal IGF-II allele is expressed.
- If the paternal allele is non-functional, the mice are small.
- Some autosomal gene pairs are functionally non-equivalent due to imprinting.
What is the role of epigenetic modifications in imprinting?
establish differences between maternal and paternal chromosomes
How are histone modifications relevant in epigenetics?
- Acetylation (e.g., by histone acetyltransferases) → Increases gene expression. (euchromatin)
- Methylation (e.g., histone lysine methyltransferases) → Leads to gene silencing. (heterochromatin)
How does DNA methylation occur and what does it lead to?
- Occurs in CpG islands (Cytosine-Guanine-rich regions upstream of genes).
- Unmethylated CpG → Active transcription.
- Fully methylated CpG → Silenced gene.
What is a CpG island?
Just the Cs and Gs before the protein sequence- almost like in the promotor region
- used because they are easy to measure
What are the differences between imprinting patterns in maternal and paternal genes?
- Maternal imprinting: Maternal allele is silent, so phenotype depends on the paternal allele.
- Paternal imprinting: Paternal allele is silent, so phenotype depends on the maternal allele.
What is an example of opposite imprinting?
A hormone gene is imprinted in the paternal genome, while its receptor gene is imprinted in the maternal genome.
What is meant by bimaternal and bipaternal?
- two female genomes (bimaternal) or two male genomes (bipaternal).
How did scientists attempt to make bimaternal mice?
- Oocyte activation (possibly using electrical stimulation).
- Extract haploid embryonic stem cells.
- Induce global DNA methylation in culture until they resemble primordial germ cells.
- Delete paternally imprinted loci one by one using CRISPR-Cas9.
- Fertilize the oocyte with another female’s nucleus and implant into a foster mother.
How did scientists attempt to create bipaternal mice?
- Inject sperm into an enucleated oocyte.
- Isolate androgenetic haploid embryonic stem cells.
- Delete every maternally imprinted region.
- Co-inject edited stem cell + sperm into a new enucleated oocyte.
- Implant into a foster mother.
What were the findings from bimaternal mice?
- 1 KO gene = severe retardation and death after birth
- 2 KO gene = growth retardation, lower cholesterol, reduced activity
- 3 KO gene = normal growth + cholesterol + normal behaviour
What are the challenges and issues with the bimaternal and bipaternal mice experiment?
- Female ESCs demethylate faster than paternal ones, requiring different culture conditions.
- Deletion of essential imprinted genes often led to developmental failure or abnormalities.
